There are many situations in which information pertaining to the color or wavelength of light illuminating a detector is of interest. Color or wavelength sensing techniques are applicable to a variety of industries, ranging from agriculture to the design and fabrication of electronic components.
In the agricultural field, the degree to which fruit has ripened may be determined based on its color and can therefore be used to determine the optimal time for picking the fruit. Similarly, in industrial settings it is often desirable to sort objects based on their color. In this way color sensing capabilities may become important to the development of robotic vision systems and other disciplines where image processing is used to extract information.
Other applications of color or wavelength sensing include instances where it is desired to obtain spectrometric data, and situations where wavelength-multiplexing (as in optically based communication systems) may result in improvements to the information transfer capability of a system. Additional uses of color sensing techniques include quality inspection, detection of subtle changes in the color of stained cell and tissue samples for purposes of medical diagnosis, and process control during the deposition of thin films in the semiconductor industry.
The wavelength of light emitted by a source can be determined by using a monochromator/spectrometer and a suitable detector. A monochromator is essentially a box which contains a diffraction grating. The incoming light is diffracted through an angle which depends on its wavelength. This allows the wavelength to be inferred based on the location at which the light is detected. The instrument must be precisely aligned with the light source and the detector in order to operate properly. The disadvantage of such instruments is that they are bulky and expensive, and are not suited for use in environments where they would be subject to vibrations.
The conventional method for detecting color is somewhat different. Color, as perceived by the human eye, is based on the three primary colors, red, green, and blue. Other colors can be generated by mixing light of these wavelengths in varying proportions. Typically, color detection is performed by using three filters, one for each of the primary colors, and three photodetectors. The filters allow the transmission of one of the primary wavelengths, and the corresponding photodetector measures the intensity of the incident light at that wavelength. The responses of the photodetectors are then used to determine the relative contribution of each of the primary wavelengths to the incident light, and hence to infer its color. A drawback to such methods is that the use of three sensors with their corresponding filters can become complicated, and take up excessive space when forming a color sensing array.
In general terms, a practical color sensing system should be able to convert detected wavelength information into electrical signals which can be subjected to further processing. This can be accomplished in a variety of ways. One method is to use charge coupled devices (CCDs) which are arranged in an array and used with color-beam splitters. However, such systems are relatively expensive and bulky, which limits their usefulness for some applications.
Another method for detecting the color of incident light is based on using photodetectors equipped with polymer dye color-filters. A disadvantage of this type of sensor is that its fabrication cannot be easily integrated with conventional integrated circuit fabrication processes. Compatibility and ease of integration with conventional integrated circuit fabrication is desirable for at least two reasons. It allows the sensor to be fabricated on a semiconductor chip using well developed, existing processing technologies. This results in cost savings as the development of new process steps can be both time consuming and expensive. In addition, the sensor and its associated circuitry can then be fabricated on the same chip. This permits miniaturization and improves the reliability of the device.
To overcome the disadvantages of using multiple detectors, a single sensor capable of detecting multiple colors has been sought. Several candidate devices have been studied, including photodiodes formed in single-crystalline silicon and those formed in amorphous silicon. The operation of these devices is based on the intrinsic wavelength filtering property of silicon which results from the variation of the material's absorption coefficient with the wavelength of an illuminating photon. The absorption coefficient is the reciprocal of the penetration depth, which is a measure of the distance traversed by a photon into a material before it is absorbed by an electron.
If the total photocurrent produced by a detector in response to incident light is modeled as a linear combination of the photocurrent due to the detection of the three primary colors, then the problem becomes one of determining the incident photon flux .phi. which corresponds to each of the primary colors. This provides an indication of the relative contribution of each of the primary colors to the color of the incident light. The photon flux can be determined by making photocurrent measurements at three values of the reverse bias voltage, thereby forming a set of simultaneous linear equations. This set of equations can be expressed as: EQU J=qS.phi.,
where J is a vector representing the three photocurrent components, q is the charge of an electron, .phi.is a vector representing the three flux components, and s is a matrix which characterizes the spectral response of the detector for the chosen values of the reverse bias voltage.
The S matrix can be independently determined by using monochromatic light of known intensity corresponding to the three primary colors, and measuring the photocurrent produced by the detector at the same three values of the reverse bias voltage. To determine the color of incident light, photocurrent measurements are made at the specified reverse bias voltages, giving the vector J. If the matrix equation for J is then solved for .phi., the color of the incident light can be determined from the relative contributions of the flux terms for the three primary colors.
Unfortunately, in order to solve the matrix equation and determine the flux terms, the matrix S must inverted. If the matrix is ill-conditioned with respect to finding its inverse, the solution for the flux terms may not be obtainable, or may be in error. This would produce an inaccurate result for the color of the incident light. This problem suggests that a more mathematically robust method for determining the color of incident light based on dc photocurrent measurements would be desireable.
A system for detecting color which uses a single photodetector and which relies on the intrinsic wavelength filtering property of a semiconductor is disclosed in U.S. Pat. No. 4,749,85, issued to R.F. Wolffenbuttel. This method is relatively complex and relies on specialized test equipment. However, its greatest disadvantage is that the method can introduce a significant degree of error in the determination of the color of incident light.
Another type of color sensor is one developed by the Nagoya Municipal Industrial Research Institute of Nagoya-shi, Japan. This sensor is based on an electrically controlled birefringence liquid crystal cell which is mounted on a p-n junction photodiode. The device indicates the color of an illuminating photon by the variation in the output current waveform of the transducer which occurs as the voltage across the liquid crystal cell is changed. The ability to electronically tune the spectral transmittance of the sensor also allows it to be used as a speotrometer. A disadvantage to this type of sensor is that the liquid crystal cell which it uses cannot be readily integrated onto the same integrated circuit chip as the photodiode.
Another topic of interest is one arising in the semiconductor industry, where the characterization of semiconductor materials is important during both basic research and device design and fabrication. The variation of the space charge width with reverse bias voltage, the dopant density, and the absorption coefficient are fundamental parameters of a semiconductor and help to determine its utility and potential range of applications.
The absorption coefficient is usually determined by comparing the intensity of incident light on a material to that transmitted through it. After corrections for the light which is reflected, the extent of light absorption can be determined, and from this the absorption coefficient can be inferred. The depletion width of a space charge region in a semiconductor can be determined by measuring the capacitance of the depletion region as a function of voltage for a sample of the material. This allows the sample's doping density to be determined, as it depends upon the depletion width. A disadvantage to both of these methods is that they require specialized instruments which can sometimes be expensive, and may not be readily available.
What is desired is a method for accurately determining the wavelength or color of light incident on a detector which is based on the use of a single detector and can be readily integrated into the standard fabrication processes used in the semiconductor industry. It is also desired to have a method for characterizing the properties of semiconductor materials which is simpler and requires less specialized instrumentation than the methods currently being used.